1. Field of the Invention
This invention pertains generally to high-frequency coupling devices, and more particularly to microwave couplers utilizing artificial composite right/left-handed transmission lines.
2. Description of Related Art
Couplers are used in circuits to generate separate signal channels with desirable characteristics. Conventional couplers may be divided into two categories: coupled-line couplers (backward, forward) and tight-couplers (e.g., branch-line, rat-race, and so forth). While the former are limited to loose coupling levels (typically less than −3 dB) because of the excessively small gap required for tight coupling, the latter are limited in bandwidth (i.e., typically less than 20%).
Coupler designs currently in use suffer from a number of shortcomings. For example, a coupler referred to as the “Lange coupler” can be classified mid-way between the two categories of coupled-line couplers and tight-couplers, yet it has the short-coming of requiring cumbersome bonding wires. The Lange coupler is described in the paper “Interdigital Stripline Quadrature Hybrid”, from IEEE Trans. Microwave Theory and Technology, volume MTT-26, pp. 1150-1151, published December 1969, incorporated herein by reference.
Conventional hybrid rings, often referred to as rat-race couplers, have the shortcomings of narrow bandwidth and large size.
Conventional branch-line couplers (or quadrature hybrids) are characterized by repetition of their coupling characteristics at odd harmonics of the design frequency. Since it is unlikely that a dual-band application would require exactly f0 and 3 f0, this coupler is therefore virtually limited to single-band operation at f0.
Accordingly a need exists for high-frequency coupling devices which provide increased flexibility with regard to type of coupling and harmonic frequency while being amenable to embodiment in compact forms.